1. Field of the Invention
The invention relates to the conversion of synthesis gas to hydrocarbons. More particularly, it relates to the conversion of such synthesis gas to C.sub.5.sup.+ hydrocarbons suitable for use as liquid motor fuels.
2. Description of the Prior Art
It is well known in the art that synthesis gas, i.e., hydrogen and carbon monoxide, can be converted to hydrocarbons in the presence of a variety of transition metal catalysts. Thus, certain Group VIII metals, particularly iron, cobalt, ruthenium and nickel, are known to catalyze the conversion of CO and hydrogen, also referred to as syngas, to hydrocarbons. Such metals are commonly called Fischer-Tropsch catalysts. While the use of nickel preferentially produces methane upon conversion of syngas, the use of iron, cobalt and ruthenium tends to produce hydrocarbon mixtures consisting of hydrocarbons having a larger carbon number than methane, as determined by a number of analytical means including mass spectrographic analysis of individual components and the boiling point curve method. At higher reaction temperatures, all Fischer-Tropsch catalysts tend to produce gaseous hydrocarbons, and it is readily feasible to select processing conditions to produce methane as the principal product. At lower temperatures, and usually at higher pressures, however, iron, cobalt and ruthenium produce hydrocarbon mixtures consisting of larger hydrocarbons. These products usually contain very long straight-chain hydrocarbon molecules that tend to precipitate as wax. Such wax material, boiling well beyond the boiling range of motor fuels, typically constitutes a significant fraction of the product produced in such catalytic conversion operations. Fischer-Tropsch catalysts have not been advantageously employed in the production of liquid hydrocarbon motor fuels, therefore, instead commonly producing either principally gaseous hydrocarbons, on the one hand, or hydrocarbons containing an unacceptably large amount of wax on the other. In addition, the gasoline boiling hydrocarbon fraction that has been produced has an unacceptably low octane number.
In light of such circumstances, efforts have been made to improve the performance of Fischer-Tropsch catalysts for use in various desired syngas conversions. For example, the Breck et al. patent, U.S. Pat. No. 3,013,990, discloses the use of zeolitic molecular sieves containing a Fischer-Tropsch catalyst as improved catalyst compositions. Thus, Type A, X and Y molecular sieves loaded with iron or cobalt are shown to be suitable Fischer-Tropsch hydrocarbon synthesis catalysts, as for the production of methanol from syngas. Also with respect to the conversion of syngas, Fraenkel et al., U.S. Pat. No. 4,294,725, teach that zeolites A and Y loaded with cobalt, incorporated by ion exchange and reduced in-situ with cadmium, serve as useful catalysts of the Fischer-Tropsch type. Those skilled in the art will appreciate that such catalyst materials tend to be relatively expensive and, in any event, do not produce hydrocarbon products advantageous for use as liquid motor fuels.
Efforts have also been made to improve Fischer-Tropsch catalyst performance by preparing intimate mixtures of Fischer-Tropsch metals, such as iron, with an acidic crystalline aluminosilicate, such as ZSM-5. The Chang et al. patents, U.S. Pat. Nos. 4,086,262, and 4,096,163, disclose such catalyst compositions employed in the conversion of synthesis gas to hydrocarbon mixture useful in the manufacture of heating fuels, gasoline, aromatic hydrocarbons and chemical intermediates. When it is desired to convert syngas specifically to hydrocarbons boiling in the jet fuel+diesel oil boiling range, however, such an approach is not suitable, experiencing an effective limitation at C.sub.10 carbon number as was the case using ZSM-5 in methanol conversion, as disclosed in the Owen et al. patent, U.S. Pat. No. 3,969,426.
While iron is the currently preferred Fischer-Tropsch catalyst component for use in syngas conversion operations, cobalt had originally been preferred because of its various desirable properties. Thus, cobalt has a higher level of catalytic activity in syngas conversion operations as well as a better selectivity to total motor fuels than is obtained using iron. One very objectional characteristic of cobalt, however, is the excessive amount of undesired methane that is produced when it is employed in syngas conversion operations, the level of methane production being considerably out-of-line with the level of other hydrocarbons produced and significantly diminishing the overall performance of said syngas conversion operations using cobalt as the Fischer-Tropsch catalyst.
It is nevertheless desirable in the art to develop improvements with respect to the use of cobalt as a Fischer-Tropsch catalyst for syngas conversion. More particularly, it is desirable to overcome the objectionable charactristics of cobalt by lowering its selectivity to methane during syngas conversion operations.
In prior art development work relating to various Fischer-Tropsch catalysts other than cobalt, the addition of copper and silver have been found to have varying effects on the selectivity of methane production. Thus, G. Bond and B. Turnham report in the Journal of Catalysis 1976, vol. 45, p. 128-136, that the addition of 50 mole % copper to a ruthenium catalyst causes the catalyst to lose significant activity and to become more selective for methanation and less selective for heavier hydrocarbon production, although one catalyst with only 3 mole % copper was found to follow the trend of the higher percentage copper catalysts, but to a lesser degree. On the other hand, D. Elliott and J. Lundsford report, in said Journal of Catalysis, 1979, vol. 57, p. 11-26, the observation of a decrease in methane selectivity upon addition of copper to a ruthenium-y zeolite composition, with this result attributed to a lower hydrogenolysis activity for the ruthenium-copper catalyst. Furthermore, J. Amelse, L. Schevarty and J. Butt report, again in said Journal of Catalysis, 1981, vol. 72, p. 95-110, that the use of an iron-copper Fischer-Tropsch catalyst containing about 25% copper based on the amount of iron therein produces more methane and less olefins than a corresponding iron catalyst without copper added thereto. The effects observed in such prior art work appear to have been dependent upon the nature of the particular Fischer-Tropsch metal component employed and upon the processing conditions employed.
It should be noted that such prior art activities relating to iron and ruthenium Fischer-Tropsch catalysts were carried out under processing conditions of high methane yield, but with varying, unpredictable results. Earlier prior art work using cobalt as the Fischer-Tropsch catalyst, however, was carried out under processing conditions of low methane selectivity, and no effect was seen with respect to said methane selectivity. Thus, the use of copper and silver in cobalt catalysts, as to reduce the reduction temperature of the cobalt, constitutes old work discussed in "The Fischer-Tropsch and Related Synthesis" by H. Storch, N. Golumbic and R. Anderson, John Wiley & Sons, N.Y. In addition, Fischer is known to have studied 9:1 and 1:1 cobalt:copper catalysts at atmospheric pressure and temperatures of about 190.degree.-220.degree. C. At these conditions, such catalysts gave quite saturated products and oxygenates. The copper was added to lower the reduction temperature of the cobalt because of equipment restrictions. No decrease in methane selectivity was observed in these experiments carried out for other purposes under conditions of low methane selectivity. Prior art experiments at I. G. Farben using a 1% silver in cobalt catalyst composition are also known to have been made, presumably at low temperature and with no noted reduction in methane yield although easier reduction and longer catalyst life were noted under th processing conditions employed. Once again, no advantageous reduction in the methane selectivity of the cobalt was either sought or observed, notably because the conditions employed for the purposes of such prior art work were such that the hydrogenolysis reaction likely leading to the production of methane as a secondary product were not employed.
Despite the variety of prior art activity referred to above, the disadvantageous characteristics of cobalt performance remain, precluding the use of cobalt as a Fischer-Tropsch catalyst, despite its outstanding activity and motor fuel selectivity when employed for syngas conversion. The desire also remains in the art, therefore, for the development of improvements enabling cobalt to be used for syngas conversion with lower selectivity for methane and a corresponding increase in selectivity for desired liquid hydrocarbon fuels.
It is an object of the invention, therefore, to provide an improved process and Fischer-Tropsch catalyst composition for the conversion of syngas to liquid motor fuels.
It is another object of the invention to provide a process and Fischer-Tropsch catalyst composition for lowering the selectivity of cobalt for methane.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.